1. Field of the Invention
The invention relates to an X-ray examination apparatus, including an X-ray source, an X-ray detector for picking up the X-ray image, and an X-ray filter which is arranged between the X-ray source and the X-ray detector, the X-ray absorptivity of the X-ray filter being locally adjustable by control of a local quantity of an X-ray absorbing liquid in the X-ray filter.
2. Description of the Related Art
An X-ray examination apparatus of this kind is known from French Patent Application FR 2 599 886.
The known X-ray apparatus is provided with a filter for limiting the dynamic range of an X-ray image, being the interval between the extremes of the brightness values. An X-ray image is formed on the X-ray detector by arranging an object, for example a patient to be examined, between the X-ray source and the X-ray detector and exposing the object to X-rays emitted by the X-ray source. If no steps are taken, the dynamic range of the X-ray image may become large. On the one hand, for some parts of the object, for example lung tissue, the X-ray transmissivity is high, whereas other parts of the object, such as bone tissue, can hardly be penetrated by X-rays. Lead shutters used to intercept parts of the X-ray beam from the X-ray source so as to shield parts of the object to be examined from the X-rays are imaged with a uniform, very low brightness. Lead shutters are also used to prevent X-rays which fail to pass through the object from reaching the X-ray detector, leading to overexposed areas in the X-ray image. If no further steps are taken, therefore, an X-ray image will be obtained with a very large dynamic range whereas, for example medically relevant information in the X-ray image is contained in brightness variations in a much smaller dynamic range; because it is not very well possible to make small details of low contrast suitably visible in a rendition of such an X-ray image, it is not very suitable for making a diagnosis. Furthermore, problems are encountered when such an X-ray image is picked up by means of an image intensifier pick-up chain. An image intensifier pick-up chain includes an image intensifier tube for converting an incident X-ray image into an optical image and a video camera for deriving an electronic image signal from the optical image. Regions of very high brightness and regions of very low brightness are formed in the optical image from regions of very high brightness and very low brightness, respectively, in the X-ray image. If no further steps are taken, the dynamic range of the optical image could be larger than the range of brightness values that can be handled by the video camera without giving rise to disturbances in the electronic image signal.
In order to limit the dynamic range of the X-ray image, the known X-ray examination apparatus includes a filter with filter elements provided with a bundle of parallel capillary tubes, each of which is connected, via a valve, to a reservoir containing an X-ray absorbing liquid which suitably wets the inner walls of the capillary tubes. In order to fill a capillary tube with the X-ray absorbing liquid, the valve of the relevant capillary tube is opened after which the capillary tube is filled with the X-ray absorbing liquid by the capillary effect. Such a filled capillary tube has a high X-ray absorptivity for X-rays which pass therethrough in a direction approximately parallel to its longitudinal axis. The valves are controlled so as to ensure that the quantity of X-ray absorbing liquid in the capillary tubes is adjusted such that filter elements in parts of the X-ray beam which pass through parts of the object of low X-ray absorption are adjusted to a high X-ray absorptivity and that filter elements in parts of the X-ray beam which pass through parts of the object of high X-ray absorption, or are intercepted by a lead shutter, are adjusted to a low X-ray absorptivity.
In order to change the adjustment of the filter of the known X-ray examination apparatus it is necessary to empty the filled capillary tubes first. Therefore, use is made of a paramagnetic X-ray absorbing liquid which is removed from the capillary tubes by application of a magnetic field. After all capillary tubes have been drained, the filter is adjusted again by deactivation of the magnetic field, followed by the opening of the valves of capillary tubes which must be filled with the X-ray absorbing liquid so as to raise their X-ray absorptivity to a high value according to the new filter adjustment.
It is a drawback of the known filter that it is not very well possible to change the adjustment of the filter within a very short period of time, say one second. Therefore, the known X-ray apparatus is not suitable for forming successive X-ray images at a high image rate, where the adjustment of the filter is changed between the formation of successive X-ray images. Because it is necessary to drain all capillary tubes before the filter elements can be adjusted to new X-ray absorptivities and because the X-ray absorbing liquid suitably wets the inner wall of the capillary tube so that draining requires a rather long period of time, amounting to several seconds or even tens of seconds, switching over the known filter requires a comparatively long period of time. Moreover, it is not very well possible to drain the capillary tubes completely by application of the magnetic field, because a layer of X-ray absorbing liquid will remain on the inner walls of the capillary tubes.
It is a further drawback of the known filter that the construction involving separate mechanical valves for each of the capillary tubes is rather complex.